Conventional golf balls generally include a core, which is encased by a cover. During use golf balls sustain repeated impacts, and those balls whose covers crack or chip are unusable. Therefore, it is critical to performance and ball life that the cover is durable.
Injection molding is a conventional method for forming the cover. According to well known techniques, injection molding utilizes a mold, which includes top and bottom mold plates, and an injection screw. Each plate defines a hemispherical molding cavity for receiving the core, a runner system for transporting a molten material, and one or more gates through which the material enters the cavity from the runner system. Generally, molds may also include pins as a centering means. Retractable pins are often used. The pins contact the core at the poles and hold the core in the cavity. The mold plates move between open and closed positions. In the open position, the mold plates are spaced apart. In the closed position, the mold plates are in contact at a mold parting plane, and the hemispherical molding cavities form a spherical molding cavity having a spherical, outer surface.
One molding cycle for forming a golf ball includes a number of steps. The core is placed within the bottom hemispherical molding cavity on the pins, and the mold plates are closed. The pins center the core in the spherical cavity during molding. Then, the injection screw forces the molten cover material through the runner system and gates into the molding cavity until the cavity is filled and the material surrounds the core. The pins begin to retract as the material comes into close proximity to the pins. The material flows and fills the apertures in the material caused by the pins. As the material cools, it solidifies in the shape of the molding cavity around the core. When the material is sufficiently cool, the formed golf ball is removed from the cavity, and the mold is made ready for another molding cycle.
When molding golf balls it is desirable that the core is centered within the cover so that the cover has a uniform thickness. It is also desirable that the core have a spherical shape. During molding, as the material enters the cavity, the material exerts forces on the core. These forces may tend to distort the core so that it is no longer spherical and/or move the core off-center. It is critical that the core is spherical and centered in the finished ball, since cores that are not spherical or off-center produce an unplayable ball.
One problem caused by the pins is that when the material contacts the pins during molding, the pins are colder than the molten material. The molten material contacts the pins and begins to solidify, and the remaining molten material may not bond properly with the solidified material. As a result, the pin sites are locations of stress concentration in the cover at the poles of the core. Moreover, the material that flows from one gate meets and joins the material that flows from another gate, and this interface forms a knit line when the material solidifies. Knit lines extend around the core and typically intersect at the poles of the core. The knit lines are weak locations in the cover. When these intersections are at the poles, where the pins also contacted the core, the stress concentrations in the cover are amplified and the likelihood of crack initiation increases undesirably.
Consequently, a need exists for an improved injection molding method and apparatus for manufacturing a golf ball. The method and apparatus should minimize deformation of the core, provide centering of the core in the cover during molding, and decrease stress concentrations in the cover.